An axial-flow fan includes a circular central hub and a plurality of blades radially arranged along the circumference of the hub, and as well known to those skilled in the art, the axial-flow fan is a kind of fluid machinery which serves to blow air in the axial direction thereof by the rotation of the plurality of the blades. A representative example of the axial-flow fan is a cooling fan that blows air for heat radiation to an air-cooled heat exchanger to promote heat radiation of the air-cooled heat exchanger, such as an electric fan, a ventilation fan, and a radiator or condenser of an automobile. The axial-flow fan that is used as the cooling fan of the heat exchanger in the air conditioning system of the automobile is mounted at the rear or front side of the heat exchanger in conjunction with a shroud that is provided with a bell-mouthed ventilating port that is surrounded around the shroud and a plurality of airflow guide vanes that serve to guide the air blown by the blades of the fan to an axial direction from the front or the rear side of the ventilating port.
The axial-flow fan may be classified into a pusher-type axial-flow fan assembly and a puller-type axial-flow fan assembly in accordance with the arranged positions with respect to the heat exchanger.
A conventional axial-flow fan 10 of an automobile is mounted in the front of the heat exchanger in conjunction with a shroud surrounding the blades of the fan and guiding air toward the axial direction. As shown in FIGS. 1 and 2, the axial-flow fan 10 includes a central hub 12 coupled to a rotational shaft of a motor (not shown), a plurality of blades 11 extending radially outwardly from the outer circumference of the hub 12, and a circular fan band 13 fixedly coupled to the peripheral ends of the plurality of blades 11 for surrounding the plurality of blades 11. The axial-flow fan is generally made of synthetic resin and integrated with the blades 11 into a single body. The plurality of blades 11 that are curved in the plane of the fan 10 are rotated as the motor is rotated, thereby producing a difference pressure according to a variation in the airflow velocity between the front and rear sides of the fan. Thus, the axial-flow fan blows air to the axial direction thereof.
Therefore, the plurality of blades 11 may have a great effect on a blowing efficiency and the amount of generated noise in the axial-flow fan 10. As shown in FIG. 8, the terms used to describe the blades 11 of the axial-flow fan 10 are defined. The axial-flow fan 10 should be designed in view of a variety of important blade designing factors, such as setting angle of the blades 11, camber ratio, cross-directional curvature, chord length, axial-directional inclination angle, leading edge line (L.E.L) and trailing edge line (T.E.L).
The camber ratio is obtained by dividing a maximum camber value into a chord length.
The setting angle is obtained by subtracting a stagger angle at which each blade 11 is erected from 90°.
According to the prior art that has been developed in consideration with the above designing factors, as shown in FIG. 3, the trailing edge line and the leading edge line of each of the plurality of blades 11 has a plurality of inflection points formed in the same direction to one another, and each inflection point of the trailing edge line and each inflection point of the leading edge line are formed on the same radius (RT3=RL3, RT2=RL2 and RT1=RL1) from the central point of the hub 12.
By the way, for the conventional axial-flow fan as constructed above, a stream of airflow is inclined to be directed straightly along an axial direction thereof, as shown in FIG. 4.
Therefore, there is a high possibility that the airflow may collide against the engine block in the engine room at a relatively high temperature. If such a situation occurs, air becomes hot and flows back to be recirculated to the front side of the heat exchanger. This makes the temperature of air introducing to the axial-flow fan substantially raised. Thus, the cooling performance of the heat exchanger becomes deteriorated.